In cellular mobile systems like GSM (but also its successors like UMTS or E-UTRAN/LTE) the so-called “PLMN Selection” (PLMN=Public Land Mobile Network) is performed by the user equipment (UE) as defined in [3GPP TS 22.011].
This basic concept of any of these cellular networks is that the distinction between different providers offering mobile services to customers is based on the unique PLMN identity of any of these networks or operators, operating a public land mobile network. These operators can be distinguished by the so-called “PLMN ID”. In Germany the PLMN ID of Telekom Deutschland is 262-01 (where 262 is the Mobile Country Code (MCC) of Germany and 01 is the Mobile Network Code (MNC) of Telekom Deutschland). Details about PLMN ID and PLMN selection can be found in [3GPP TS 22.011].
[3GPP TS 22.011] defines two principle modes for PLMN selection:                Automatic PLMN Selection Mode, and        Manual PLMN Selection Mode        
In “automatic PLMN selection mode” the selection of the public land mobile network is automatically performed by the User Equipment. It can be controlled—e.g.—by a data field contained in the SIM or USIM of the terminal equipment. These data files or data fields are either populated by the Home PLMN operator which issued the SIM/USIM card and contain a list of preferred “PLMNs for roaming” (operator controlled PLMN list). The alternative is the provision of the so-called “user controlled PLMN list” which can be edited by the subscriber to define his personal preferences for roaming with certain public land mobile networks.
Alternatively the terminal equipment/User Equipment can be operated in “manual PLMN selection mode”: In this mode a new public land mobile network is not selected automatically by the terminal equipment/User Equipment in case the old public land mobile network is lost, but an indication is given to the user on the display that a new PLMN needs to be selected in case of PLMN loss. All details are also defined in [3GPP TS 22.011].
“PLMN selection” is normally only performed in case the old PLMN, typically the Home PLMN of the subscriber is lost. A typical example is when a User Equipment crosses a country border and loses the Home PLMN of the old country. In this case, if the User Equipment is in automatic mode, the User Equipment automatically searches for a new public land mobile network in the new country according to either the “user controlled PLMN list” or the “operator controlled PLMN list”. As there are typically international roaming agreements with more than a single operator in the target country, the “operator controlled PLMN list” brings the terminal to the “partner network” in the target country. The preference of the user to select a particular PLMN in a particular country can be controlled by the mentioned “user controlled PLMN list”, which takes presence over the “operator controlled PLMN list” according to [3GPP TS 22.011].
A problem in current cellular networks is the service offering in the country border region. In particular, subscribers which live in the area of the country border might wish to avoid international roaming charges when moving out of the Home PLMN into a PLMN of another country as typically the international roaming charges are higher than the ones for the Home PLMN (which might even be included in a flat rate of the subscriber).
In order to avoid a User Equipment in the country border region automatically roaming into a PLMN of another neighbour country, subscribers living in such areas often disable the automatic PLMN selection and use “manual PLMN selection mode”.
A negative side effect of such customer behaviour is the fact that the User Equipment remains unreachable unless a new public land mobile network is manually selected by the user or the old (Home) PLMN becomes available again.
Another concept defined by 3GPP for GSM, UMTS and E-UTRAN/LTE enables the network to signal so-called “equivalent PLMNs” (ePLMNs) to a particular User Equipment in order to minimise the service outage which exists also in automatic mode and allows the operator of the source PLMN to control individually to which target PLMN User Equipments should be transferred when crossing a country border. Public land mobile networks which are defined as ePLMNs look to the UE like being the same PLMN and the UE performs a cell reselection between those ePLMN rather than a PLMN selection [see 3GPP TS 23.122].
By using this concept of ePLMNs, the operator can control the User Equipment regardless of the PLMN selection lists contained in the SIM/USIM (“user controlled PLMN list” and “operator controlled PLMN list”). The concept was mainly defined to allow cross-country provision of seamless services between public land mobile network operators belonging to the same group—e.g., to enable a German Telekom Deutschland mobile customer to seamlessly roam into the Austrian T-Mobile AT or the Dutch T-Mobile NL networks. However nowadays the ePLMN concept is also used in various network sharing scenarios within a given country.
A negative side effect of the usage of the ePLMN concept is that the “manual PLMN selection” decision by the customer is overwritten when using this concept along country border to steer international roaming customers into the preferred networks [see 3GPP TS 23.122]. By doing this the roaming charges would affect also those users which by intention disabled the automatic PLMN selection in order to avoid high roaming charges.